A technique called multi-user (MU) multiple-input and multiple-output (MIMO) has been known in which a data signal is simultaneously transmitted to multiple users equipment (UEs) at the same frequency from a base station including multiple transmission antennas. Here, a base station is an example of a transmitter station when wireless communication is focused on downlink (DL), and a UE is an example of a receiver when the wireless communication is focused on the DL.
In MU-MIMO, a technique of inter-user signal orthogonalization has been known which is exemplified by zero forcing (ZF), which multiplies a transmission signal by a transmission weight in the base station not to occur interference between UEs, and block diagonalization (BD).    [Patent Document 1] Japanese Laid-open Patent Publication No. 2015-33098    [Patent Document 2] Japanese Laid-open Patent Publication No. 2008-35288    [Patent Document 3] Japanese Laid-open Patent Publication No. 2011-30139    [Non-Patent Document 1] D. Gesbert, M. Kountouris, R. Heath Jr., C. Chae, and T. Salzer, “Shifting the MIMO Paradigm,” IEEE Signal Process. Mag., vol. 24, no. 5, pp. 36-46, September 2007.
In traditional techniques of inter-user signal orthogonalization, a receiver moving faster more easily deviates from a direction (referred to as “null”) that minimizes the gain of the beam formed by application of a transmission weight in the transmitter.
Therefore, interference may occur between UEs moving different speeds. If signal interference occurs between UEs, the throughput performance of the DL degrades, for example.